1. Field of the Invention
The present invention relates generally to RF/digital receivers, and in particular to a signal-separating configuration for GNSS multi-antenna directional receivers and a receiver manufacturing method, which provides more accurate data in a more compact and economical size than previous GNSS-based heading devices.
2. Description of the Related Art
Global navigation satellite system (GNSS) guidance and control are widely used for vehicle and personal navigation and a variety of other uses involving precision location and machine control in geodesic reference systems. GNSS, which includes the Global Positioning System (GPS) and other satellite-based positioning systems, has progressed to sub-centimeter accuracy with known correction techniques, including a number of commercial satellite-based augmentation systems (SBASs).
GNSS guidance devices currently come in a variety of forms and function in a variety of different ways. For instance, the typical commercial GNSS guidance device located in a standard vehicle contains a receiver, an antenna, a graphical interface to instruct the vehicle operator where to go, and a processor, e.g., a central processing unit (CPU), for running calculations and processing requests.
Other uses for GNSS guidance include using the GNSS device as a bearing device or directional receiver, i.e., a multi-antenna directional receiver. The GNSS system can be used to determine heading information for a host system, such as a vehicle or a piece of equipment. Typically a GNSS directional receiver has a centrally-located receiver and two or more separated antennas with low noise amplifiers (LNAs) to detect the phase differences among the carrier signals from GNSS satellites in various constellations, of which at least four satellites are visible at any given time for calculating GNSS-based position and heading fixes. Given the positions of the satellite, the position of the antenna, and the phase difference, the orientation of the two antennas can be computed. Additional antennas may be added to provide multiple readings with respect to each satellite, allowing three-dimensional (3D) position and heading solutions for the GNSS-equipped vehicle. A GNSS directional receiver is not subject to magnetic declination as a magnetic directional receiver is, and doesn't need to be reset periodically like a gyrodirectional receiver. It is, however, subject to multipath effects, which susceptibility is addressed by the present invention.
A potential performance-related receiver design problem relates to cross-coupling between the radio frequency (RF) signals from either or both of the two antennas: the master and the slave. This creates an error in the heading and position as the cross-coupled signal appears as a delay in time which smears the correlation peak and makes it more difficult to resolve the exact range to the satellite. This can also create a reduction in signal to noise ratio (SNR) if the cross-coupled signals cause a cancellation effect.
Another potential performance-related receiver design problem relates to digital signals being inherently noisy for RF as they have fast rising edges which have high harmonics content. These high harmonics can land in either the intermediate frequency (IF) or the RF frequency bands and increase the noise, thereby impairing the tracking of the desired signals. Still further, routing of the RF coaxial cables can create significant interference as they can pick up the digital harmonics and impair the signal tracking. If these signals are digital (especially low-voltage differential signal (LVDS)) they will not be as sensitive to picking up noise. Moreover, LVDSs do not generate as many emissions as normal single-ended digital signals. Different drivers exist for creating and receiving LVDSs.
The present invention addresses the RF-digital signal interference problems with previous GNSS receivers. Heretofore, there has not been available a signal-isolating GNSS receiver with the advantages and features of the present invention.